Photovoltaic solar cells often include a top contact which consists of a metal grid on top of the light absorbing layers of the device. The design of the grid is such that it must be sufficiently electrically conductive yet permit enough light to pass to the underlying cell. These requirements for conductivity and transparency are often at odds with one another. For instance, a thicker grid design leads to better conductivity but reduced transparency, and vice versa.
At high solar concentrations, the design of a photovoltaic solar cell grid is important for maximum energy conversion efficiency. The current density is very high and appreciable power can be lost in the grid resistance. Conventional grid designs often include a simple symmetrical pattern consisting of a plurality of metallic fingers connected to a common bus which is usually a larger metallic contact at the sides of the cell. For instance, a common linear grid design is to have two parallel bus connectors on opposite sides of the cell and thinner metallic fingers interconnecting the bus connectors in a ladder-like configuration. Another commonly employed design is an inverted square symmetry grid configuration. See, for example, Wen et al., “Optimization of grid design for solar cells,” Journal of Semiconductors, vol. 31, no. 1 (January 2010) (hereinafter “Wen”) (FIG. 1a shows a linear grid design and FIG. 1b shows an inverted square symmetry grid configuration).
The charge carriers generated in a solar cell must travel along the metallic fingers of the top grid until they reach the common bus. The further the carriers travel along the grid to reach the bus, the greater the resistive power loss is. With conventional grid designs the loss due to resistance is still undesirably high.
Thus, concentrator solar cell top contact grid designs that maximize both conductivity and transparency would be desirable.